The present invention relates to a treating method for bonding a cover cloth to an elastomeric material forming the body of a toothed drive belt.
Number 1 in FIG. 1 indicates as a whole a known toothed belt comprising a body 2 of elastomeric material in which are embedded a number of threadlike longitudinal reinforcing inserts 3.
Body 2 comprises teeth 4, which are covered with a cloth 5 comprising weft yarns 6 extending lengthwise of belt 1, and warp yarns 7 extending crosswise of belt
With reference to FIG. 2, each weft yarn 6 consists of an elastic core yarn 8 and at least one composite yarn 9 wound about the elastic yarn and in turn comprising a high thermal and mechanical resistance yarn 10 about which is wound at least one covering yarn 11.
The elastic yarn 8 is preferably made of polyurethane.
Yarn 10 must ensure high mechanical and thermal resistance of cloth 5 as a whole, and therefore preferably consists of paraaromatic polyamide. Moreover, having a lower abrasion resistance than covering yarn 11, yarn 10 must be isolated from the outer surface to prevent damage by the severe friction on the pulley teeth, that could result in failure of teeth 4. For this reason, covering yarn/s 11 must be of greater volume than high thermal and mechanical resistance yarn 10 so as to cover it completely, and is/are therefore preferably made of texturized polymer material, more preferably aliphatic polyamide, and even more preferably nylon 66 fibers. Alternatively, covering yarn 11 may have an hybrid composition, preferably comprising nylon and teflon, and may be single or multiple, preferably two yarns, one wound clockwise and the other anticlockwise about high thermal and mechanical resistance yarn 10.
Table 1 shows an example composition of the various weft yarns 6.
Warp 7 of cover cloth 5 differs from weft 6 described above. Its yarns are not necessarily elastic, and therefore preferably consist of polyamide, more preferably nylon 66.
An important stage in the manufacture of toothed belts is bonding the fibers of the cloth to the elastomer constituting the body and teeth of the belt. This is done using various methods, the commonest of which is to treat the cloth with a latex solution of resorcinol, formaldehyde and rubber, commonly known as RFL. The treatment consists in impregnating the fibers by immersion in a bath comprising an aqueous RFL solution.
When treated with RFL, cloth 5 described above poses problems as regards adhesion to the elastomer of the teeth, and has a low abrasion resistance.
High thermal and mechanical resistance yarns, such as aromatic polyamides, are not directly treated with RFL, but are normally preactivated, e.g. pretreated with isocyanates, polyurethanes, aliphatic epoxy resins or plasma.
Due to the complex structure of the cloth described, RFL treatment mainly acts on the covering yarns and not on the underlying high resistance yarns. In the cloth described, however, the covering yarns 11 often cover only part of the high thermal and mechanical resistance yarns 10.
RFL treatment also reduces the tensile strength of cloth consisting of aliphatic polyamide fibers, whereas the advantage of using such yarns is to increase tensile strength.
It is an object of the present invention to provide a method of treating a toothed belt cover cloth, designed to eliminate the aforementioned drawbacks, and which, in particular, does not reduce the overall tensile strength of the cover cloth, and provides for bonding the cloth with no pretreatment required.
According to the present invention, there is provided a toothed belt, wherein the toothed portion is covered with a cover cloth consisting of a weft and a warp; said weft consisting of weft yarns defined by an elastic core yarn and by at least one composite yarn wound about said elastic yarn; said composite yarn comprising a high thermal and mechanical resistance yarn, and at least one covering yarn wound about the high thermal and mechanical resistance yarn; said cover cloth covering the teeth having been treated with an adhesive composition; characterized in that said adhesive composition comprises:
a highly saturated and carboxyl groups containing nitrile and conjugated diene copolymer latex;
a resorcinol and formaldehyde resin;
aromatic epoxy resins.
According to the present invention, there is also provided a method of treating a toothed belt cover cloth, characterized by comprising the step of treating said cloth with an adhesive composition comprising:
a highly saturated and carboxyl groups containing nitrile and conjugated diene copolymer latex;
a resorcinol and formaldehyde resin;
aromatic epoxy resins.